Orthorhombic LiMnO2 products, synthesised by the reaction of gamma-MnO2 and LiOH in argon at 600-620 degrees C using carbon as a reducing agent, have been evaluated as electrode materials in lithium cells. Products that contained a minor proportion of a lithiated spinel phase showed greater electrochemical activity than pure LiMnO2. On delithiation, LiMnO2 transforms irreversibly to a spinel-type structure. A mechanism for the orthorhombic LixMnO2-spinel phase transformation is proposed.